The present invention relates to an apparatus for cooling and aerating the contents of a vessel, and more particularly to such an apparatus which is used for enhancing the biochemical action of microorganisms in such a vessel.
There are already known various arrangements for biochemically treating a substrate, such arrangements being used either for fermentation of the substrate, which is usually accomplished under anaerobic conditions, or for growing microorganisms, which is usually accomplished under aerobic conditions and where the substrate constitutes a nourishing medium. The present invention is concerned with the latter type of arrangements, especially such where the substrate is a body of liquid medium.
In the arrangements of this type, which employ biochemical action vessels in which the substrate is accommodated and acted upon by the microorganisms, there are encountered several problems. First of all, it is necessary to introduce substantial quantities of air into the substrate and to thoroughly mix the air with the substrate so as to enhance the biochemical action of the microorganisms and thus their growth. On the other hand, the biological action of the microorganisms results in liberation of substantial amounts of thermal energy, the biochemical reactions being exothermic, which thermal energy must be removed from the substrate in order not to interfere with the further growth of the microorganisms. Both of these problems must be solved in order to present a functioning biochemical action vessel of this type, and it has been heretofore difficult to solve these problems without involving excessive expenses in terms of capital investment and operating costs. This is due to the fact that the introduction of air into the substrate, as well as the deprivation of the latter of heat, required substantial amounts of energy. These increased costs are especially pronounced when the biochemical vessels are relatively large, in which case quantities in the order of several hundred cubic meters of air must be introduced into the vessel per minute, so that the air compressors used for delivering these substantial amounts of air must have a power input in the order of several thousand kilowatt, while cooling systems must be employed which have several thousand square meters of cooling area. All this considerably contributes to the significant cost of constructing and operating such an arrangement.
Various devices have already been proposed which are so constructed as to assure that the substrate will be sufficiently aerated while a sufficient amount of heat will also be removed from the substrate so that the biochemical reactions will be conducted under optimum conditions. Such devices are finding an ever-increasing application in the field of producing nutrient and fodder proteins, that is in the field of growing yeasts, bacteria and fungi. In the heretofore known devices, the problems of aerating the substrate on the one hand, and of removing heat from the substrate on the other hand, are for the most part solved independently of one another. Thus, for instance, there are already known high-output biochemical vessels in which the heat-removal process is accomplished outside the vessel in that the fluid medium or substrate is conducted to a heat exchanger located outside the vessel, while the substrate is supplied and mixed with the air needed for the biochemical reaction directly in the vessel. However, experience has shown that this is disadvantageous for at least two reasons: first of all, the microorganisms which are present in the substrate and carried with the same into the heat exchanger suffer in the latter an acute deprivation of oxygen, and secondly, circulation pumps are needed which have a very high throughput and, consequently, a high power consumption. On the other hand, the introduction of the air into the biochemical action vessel proper requires a high compressor output in terms of the quantity of the air delivered and the pressure differential to be overcome and, consequently, a high power input to such a compressor. This latter is due to the fact that the compressor has to overcome a relatively high static pressure of the substrate contained in the vessel, particularly since the air is usually introduced into the vessel in the bottom region thereof in order to assure sufficient aeration of the entire body of the liquid in the vessel.
Another disadvantage of the conventional arrangement of this kind is that the heat exchanger must be so constructed as to have several thousand square meters of surface area. This results in some other disadvantages, in addition to the substantial cost of such a heat exchanger. Thus, for instance, in many of the arrangements of this type a plurality of pipes is employed which connect the interior of the vessel with the exterior thereof and which also serve for conducting the fluid which is to exchange heat with the environment of the heat exchanger. With such an arrangement, it is extremely difficult to locate a leak if such should occur. Also, it is very difficult to periodically clean the heat exchanger so as to remove deposits of the microorganisms or other substances from the same.